U.S. Pat. No. 3,440,894 to Hammann et al. contains a good description of traction and the tractive drives to which our invention pertains.
As stated therein, traction is broadly defined as the adhesive friction of a body on a surface on which it moves. Tractive devices, as considered in light of the present invention, are those devices in which torque is transmitted through nominal point or line contact, typically with a rolling action. Although sometimes referred to as friction drives, such devices are more appropriately described as tractive drives. A tractive drive, in simplified form, could comprise two parallel cylindrical rollers in tangential contact, one roller being the input member and the other the output member. The torque capacity of such a tractive drive is a direct function of the contact pressure between the rollers and the coefficient of traction of the roller surfaces. The phrase "coefficient of traction" is preferred instead of "coefficient of friction" in order to connote rolling contact.
The traction existing at the rolling contacts of ball and roller bearings, although considered detrimental in most applications, can be used to transmit tangential force. If the coefficient of traction and the normal load on the rolling bodies are sufficient to prevent slipping, any ball or roller bearing can serve as a prototype of a tractive drive.
As further stated by Hammann et al. a distinguishing feature of tractive drives is that torque is generally transmitted therein from one member to another member by traction generated through nominal point or line contact. This is in contrast with a true friction drive such as an automotive friction clutch or a belt drive where torque is transmitted through area contact. When point contact or line contact are referred to herein, the term nominal is employed to signify that the actual contact area is something greater than that of a point or a line.
Also, as stated by Hammann et al, to further distinguish tractive drives from friction drives, consider the definition of friction. Friction is defined as the resistance to relative motion between two bodies in contact. In friction drives, a high resistance to relative motion is desired. Thus, advantage is taken of the resistance to relative motion of two or more bodies in contact to provide means for transmitting torque. In friction drives, therefore, it is desirable, under stabilized speed conditions, to have a high coefficient of friction, thus minimizing or avoiding any rolling or sliding contact between members; whereas, tractive drives, as defined above, intentionally incorporate some form of relative motion between the load carrying members, and this relative motion is not in the form of slippage. A familiar example of a friction drive comprises an input member and an output member having mating wedged surfaces wherein the surface material possesses a high coefficient of friction. The surface material can be fiber, asbestos, leather, etc. Increased torque capacity is achieved therein by tighter wedging of the members, the latter being engaged with area contact and having no slippage or relative motion therebetween.
Dimers of styrene or methylated styrenes (where the methyl group is on the side chain or the ring) are well known. See for example Hammann et al. U.S. Pat. No. 3,411,369 which discloses a hydrogenated cyclic dimer of a .alpha.-methylstyrene as a traction fluid. Stahly U.S. Pat. No. 3,272,879 has an incidental disclosure of such dimers as power transmission fluids although the term power transmission fluids covers a large multitude of applications in all of which the traction characteristics of the fluid are irrelevant, such as power steering, automatic transmissions, fluid drives, rotary vane pumps, power brakes, aircraft landing gears, industrial presses, excavating equipment and so on. F. G. Rounds in JOURNAL OF CHEMICAL AND ENGINEERING DATA, October 1960, Vol. 5, No. 4, pgs. 499 et seq. discloses naphthenic mineral oils as having better friction characteristics than paraffinic or mixed oils. However, the author points out that he was unable to relate friction characteristics with specific hydrocarbons.